Bearing Pad to Pressure Tube Contact Simulation
32nd Annual CNS Conference - 2011 June 05-08


Presented at:
32nd Annual CNS Conference
2011 June 05-08
Location:
Niagara Falls, Canada
Session Title:
Thermalhydraulics

Authors:
Farshad Talebi (PhD Student)
Azin Behdadi (PhD Student)
John C. Luxat (McMaster University)
  

Abstract

Thermal creep strain deformation is a very important pressure tube failure mechanism. During a postulated LOCA (loss of coolant accident) with failure of emergency core injection sys- tem (ECIS), the fuel cladding temperature rapidly increases and the pressure tube becomes completely dry in a few seconds after flow stagnation occurs. Subsequently, the pressure tube circumference is heated by thermal radiation except at the spots where the bearing pads are in direct contact with the pressure tube. Therefore, the localized hot spots are developed on the pressure tube’s inner surface under the bearing pads. The main objective of this paper is to eval- uate the local thermal-mechanical deformation of a pressure tube in a CANDU reactor and to investigate the fuel channel integrity under localized contact between bearing pad and pressure tube. Furthermore, the mechanistic models are validated against the experimental works per- formed at WRL (Whiteshell research laboratory). Calculations are performed using the finite element method in which the heat, thermal mechanical and creep strain equations are solved, simultaneously. According to the experimental set up, the heat conduction from bearing pads to the inner surface of the pressure tube with appropriate convective and radiation boundary condi- tions has been simulated. Furthermore, the thermal creep strain deformation has been obtained for when the pressure tube is still under operational condition. It is observed that the pressure tube thermal strain will occur if sufficient high temperature is reached however, depending on the severity of flow degradation in the fuel channel, these localized hot spots could represent a potential creep strain failure of the pressure tube. Whether the pressure tube would fail at these hot spots before contacting the calandria tube depends on the localized temperature and expe- rienced pressure transients. Sensitivity analysis is performed in order to evaluate the contact conductance, the contact width and zircaloy surface emissivity. The pressure tube local strain is very sensitive to these parameters.

 

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